On the stator blades in the last stage of very large steam turbines, a supersonic flow predominantly prevails on the outlet side. The influence of the flow angle, which is customarily brought about as a result of the incidence angle of the stator blades, in this case is determined to a great extent as a result of the supersonic expansion and the shock fronts. In addition, the design engineer is confronted with the situation that the stage reaction degree is undesirably subjected to sharp radial changes. There are different ways of influencing these parameters, such as by applying a twisting of the blade profile over the blade height (“twist” in English), an inclination of the blade airfoil in the tangential or circumferential direction (“lean” in English), and the use of special supersonic profiles. Furthermore, the blade airfoil can have a sweep, or inclination in the meridional plane (“sweep” in English), in order to influence the flow. The latter inclination is typically specified by a sweep angle in the meridional plane (“sweep angle”) which is included by a radial to the rotor axis and by a tangent which lies against the blade airfoil in the meridional plane, wherein the sign of the angle measured from the radial is critical. However, other inherently dependent flow parameters are also altered in this case. In the case of large steam turbines, large variations of the hub radius and casing radius additionally lead to special flow structures which are to be implemented with a local inclination and incidence of the blades only with difficulty in order to achieve the set aims with regard to the radial flow angle variation and reaction degree variation.
For increasing the efficiency in the case of the stator blades in the last stage of a steam turbine, it has been proposed in U.S. Pat. No. 5,906,474 to provide rounded corners at the transitions between the blade airfoil and the base.
In contrast, in WO-A1-2005/005784 it is proposed to provide the turbine blades (both stator blades and rotor blades) with a sweep which is negative on the rotor-side and stator-side edge zones of the blade, and which changes into positive in the middle region in between.